Genetic studies with AKR mice have shown that the Fv-1b gene strongly inhibits the development of thymomas. This gene restricts the expression and spread of N-tropic viruses including the endogenous AKR ecotropic virus and N-tropic MCF viruses. Congenic AKR. Fv-1b mice express about 10- to 100-fold less ecotropic virus in the thymus than AKR mice, and do not develop thymosas at 6 to 12 months of age like their AKR counterparts. To study whether the Fv-1b effect in vivo is mediated by thymocytes or thymic stroma, reciprocal subcutaneous thymus graft and bone marrow chimeras are being constructed between AKR/Cum and AKR. Fv-1b mice. The former strain is Fv-1n and carries the Thy 1.2 marker on thymocytes, allowing one to distinguish thymocytes derived from AKR/Cum bone marrow in AKR.Fv-1b recipients and vice versa. Preliminary results indicate that thymic stroma does not mediate the restrictive Fv-1b effect in mice carrying AKR and AKR. FV-1b subcutaneous thymic grafts. Further experiments involving bone marrow chimeras are in progress.